Helen Mark visits Coleshill in Oxfordshire to learn about its wartime secrets. In 1940, with fear growing that Britain could be invaded by the German army, the estate became the training headquarters for a secret underground army. Over the next four years, thousands of country men - such as farmers, gamekeepers and foresters - were trained in underground resistance. They lived outwardly ordinary lives, but their job was to spring into action in the event of invasion, disappearing into bunkers buried in the landscape and emerging to disrupt the invading army through sabotage and hand-to-hand combat. Their life expectancy would have been around two weeks.

With its quiet countryside location, far from military targets but near good transport links to London, Coleshill was the perfect place for this top-secret training base. High walls around the estate also kept its activities shielded from prying eyes. Even after the war the cloak of secrecy persisted, and today most people have never heard of the role Coleshill played in Britain's wartime history.

Helen climbs down into a replica of the original underground operational base, used for training recruits, and finds out what life would have been like for these 'Auxiliary Units' or 'Auxiliers', as they were known. Sworn to secrecy, many never spoke of their experiences and took their knowledge with them to the grave. The feared German invasion never happened, so their services were not called on for real, and in many cases even their families never knew what the Auxiliers had signed up for. Now many people are piecing together their family histories and are keen to find out whether their fathers, grandfathers or uncles may have been part of one of the best-kept secrets of World War II.

Produced by Emma Campbell